The background of and an elucidation of the problems in connection with corrosion products derived from the primary heating system of nuclear reactors are closely described in U.S. Pat. No. 4,704,235 (corresponds to Swedish Publication No. 451,915 and U.S. Pat. No. 4,704,235. Said patent specification also discloses a method by means of which many of the problems within this area are eliminated or at least substantially reduced. Said method is especially adapted for use in operating and maintaining working plants of the pressurized water reactor type. The present invention represents a development of the method referred to, where the invention has been shown to give an improved decontamination effect as well as the possibility of obtaining a final product that is less environmentally harmful or more suited to be deposited than the final product disclosed in the above-mentioned Swedish patent specification. In this context, it has turned out that the invention is such effective and advantageous that it is especially well suited for the decontamination of reactors in connection with an ultimate demolition thereof or a scrapping of spent components thereof.
A practically useful and accepted method of last-mentioned type is definitely desired in Sweden today. Thus, the Swedish nuclear plants comprise reactors which were started between 1972 and 1985. A natural consequence thereof is that the requirements for maintenance and repairs of system components will continue to increase. Eventually some of these components have to be replaced. Replacements have already started of a number of large components, such as preheaters, moisture separators, etc., at some of those plants which were started first.
The replaced components can either be transferred to SFR for an ultimate deposit, optionally after some intermediate deposit in the plants, or be conditioned for example to make possible a free-classification/recycling of material. If the latter alternative is chosen, which is the preferred one if one wants to minimize the total volume of waste to be sent to the ultimate deposit, there will for instance be a great demand for decontamination methods giving high decontamination factors (DF). In addition thereto it must be possible to take care of the secondary waste obtained in an acceptable way. It has been found that the method according to the invention gives a solution to said problem.
In this context it can be added that today a number of "hard" decontamination methods are available but that generally these methods are characterized by several treatment steps, which for instance means that large amounts of chemicals have to be taken care of. Furthermore, many of these chemicals are difficult to treat.
The method according to U.S. Pat. No. 4,704,235 is based on an exposure of the contaminated surfaces or oxides to an oxidation agent in an acid solution, which oxidation agent is a combination of Ce.sup.4+ ions, ozone and chromic acid, nitric acid being specifically mentioned as the most effective and suitable acid. The present invention is based on principally the same oxidation components, i.e. Ce.sup.4+, ozone and chromic acid, the oxidation, however, being performed under different acid conditions than according to the prior art, which has been found to give essential advantages for many purposes.
U.S. Pat. No. 4,657,596 discloses the use of a decontamination agent which may contain a perhalogen acid, but said decontamination agent does not comprise all components which are required according to the present invention to obtain a synergistic effect. Furthermore, U.S. Pat. No. 4,657,596 does not disclose or even suggest that a perhalogen acid might be better than any of the other acids mentioned. Rather, the best decontaminating factors are obtained by means of an agent based on sulphuric acid.